The present invention relates to a method for remotely calibrating a phased array antenna and more specifically for active phased array antennas.
In a phased array antenna system, accurate measurement and correction in a calibration process of amplitude and phase errors is necessary for low side lobe array designs. Amplitude and phase errors may be caused by array deformation, active element pattern errors, phase shifter errors, or insertion phase and amplitude errors from electrical mismatches or tolerances in array components. The errors may also be independent among array elements, correlated over a sub-array of elements, and correlated over a row or column of elements.
Array deformation errors are actually element position errors that cause a phase error that depends upon the scan angle. This type of error can be corrected for all scan angles simultaneously.
Phase shifter, or phase quantization, errors are scan independent and contribute to side lobe level and beam pointing accuracy. For a 5 bit phase shifter, the phase shifter error has a root-mean-square (rms) value of 3.25xc2x0. This error is known a priori.
Insertion phase errors are random errors in the phase shifters or radiating antenna elements that may vary with time. In a phased array, random errors in the element path may also be incurred from active devices such as amplifiers, mixers, etc.
For satellite phased arrays, conventional calibration techniques are affected by propagation errors and satellite rotation. In all arrays, some techniques change the array environment for each measurement, and so create mutual coupling variations in the array environment.
Implementation of a phase measurement method on the active array requires a coherent phase reference signal. This signal must be coherent with the signal received at the null location of the measurement probe. The obvious method of providing this reference would be to split the source signal input to the array to provide a separate coherent reference signal. This method is not practical because of the required modification of the array hardware. It is desirable if possible to devise a method which is independent of the existing hardware, and can be simply implemented as a stand-alone measurement system, and which measures only a single antenna element path even though all antenna elements are active during the process of remotely calibrating a phased array antenna.
It is an object of this invention to provide an apparatus and method for remotely calibrating a phased array antenna, which yields the ability to measure only a single phase-shifter, even though all phase-shifters are active during the measurement.
In the present invention, for the transmit mode, a system is disclosed for calibrating at least one element of a transmit array antenna, the array antenna being coupled to a signal generating means for generating a coherent reference phase signal to at least one element, the signal emerging from the signal generating means transmitted as a beam from the array antenna, and the beam produces a null and a focused difference pattern having peaks on either side of the null. The array antenna forms a finite plane of the elements, the plane including a center point and extreme edges. The system for calibrating the elements of the array antenna comprises a null probe including a face, with the null probe being rigidly positioned normal to the plane of the array antenna and at a distance in the near-field of the array antenna, and the face of the null probe is rigidly positioned over the center point of the plane thereby forming an angle between the center point and a point on one of the extreme edges of the plane of the array antenna. The null probe is therein positioned over the center point of the array antenna corresponding to the null, so that the null probe, the center point, and the point on the extreme edge of the plane of the array antenna thereby form a plane normal to the finite plane of the elements.
A reference probe including a face is rigidly positioned parallel to, and at an offset position away from, the null probe in the plane formed by the null probe, the center point and the point on the extreme edge of the plane of the array antenna. The reference probe is also rigidly positioned at the distance in the near-field of the array antenna such that the face of the reference probe is positioned over one of the peaks of the focused difference pattern of the null. The null probe receives at the center point of the array antenna the null beam transmitted by the array antenna from the signal generating means to at least one element, and the beam received by the null probe is coupled as a signal to a first attenuator.
The first attenuator is coupled to a first two-way splitter, the first two-way splitter receiving the signal from the null probe through the first attenuator. The reference probe receives at the offset position from the null probe the beam transmitted by the array antenna from the signal generating means, and the beam received by the reference probe is coupled as a signal to a second attenuator. The second attenuator is coupled to a second two-way splitter, the second two-way splitter receiving the signal from the reference probe through the second attenuator.
A first mixer is provided which includes as input ports a radio-frequency port and a local oscillator port, and including as an output port an intermediate frequency port. A second mixer is provided which also includes as input ports a radio-frequency port and a local oscillator port, and including as an output port an intermediate frequency port.
The first two-way splitter splits the signal received from the null probe through the first attenuator into a first signal, substantially 0 degree phase-shifted, which is coupled to the radio-frequency port of the second mixer, and into a second signal, substantially 90 degree phase-shifted, which is coupled to the radio-frequency port of the first mixer. The second two-way splitter splits the signal received from the reference probe through the second attenuator into a first signal, substantially 0 degree phase-shifted, which is coupled to the local oscillator port of the second mixer, and into a second signal, substantially 0 degree phase-shifted, which is coupled to the local oscillator port of the first mixer.
The first mixer combines the substantially 90 degree phase-shifted signal of the first two-way splitter and the substantially 0 degree phase-shifted signal of the second two-way splitter to yield an output signal at the intermediate-frequency port of the first mixer. The output signal at the intermediate-frequency port of the first mixer is coupled to a first low-pass filter from which emerges a quadrature base-band component signal at substantially 0 degree frequency of the signal received from the null probe.
The second mixer combines the substantially 0 degree phase-shifted signal of the first two-way splitter and the substantially 0 degree phase-shifted signal of the second two-way splitter to yield an output signal at the intermediate-frequency port of the second mixer. The output signal at the intermediate-frequency port of the second mixer is coupled to a second low-pass filter from which emerges an in-phase base-band component signal at substantially 0 degree frequency of the signal received from said null probe.
A processor receives as an input signal the quadrature base-band component signal at substantially 0 degree frequency of the signal received from the null probe and also the in-phase base-band component signal at substantially 0 degree frequency of the signal also received from the null probe. The processor processes the quadrature base-band component signal and the in-phase base-band component signal into data representing calibration reference values of at least one element of the array antenna. A recorder records the data representing calibration reference values of at least one element of the array antenna.
Finally, a beam steering controller selects the phases on at least one element of the array antenna to form the difference null, and to vary the phase of at least one element of the array antenna.
For the receive mode, the present invention discloses a system for calibrating at least one element of a receive array antenna, with the array antenna forming a finite plane of the elements, the plane having a center point and extreme edges. The system in the receive mode comprises a signal generating means for generating a coherent reference phase signal, which is coupled to a signal source two-way splitter, the signal source two-way splitter splitting the coherent reference signal into a null signal carried to a horn probe. The horn probe includes a face, and the horn probe is rigidly positioned normal to the plane of the array antenna and at a distance in the near-field of said array antenna. The face of the horn probe is rigidly positioned over the center point of the plane thereby forming an angle between the center point and a point on one of the extreme edges of the plane of the array antenna. The horn probe is therein positioned over the center point of the array antenna corresponding to the null. The horn probe, the center point, and the point on the extreme edge of the plane of the array antenna thereby form a plane normal to the finite plane of the elements. The horn probe transmit at the center point of the array antenna the null beam received by the array antenna from the signal source to at least one element. The null beam received by the array antenna is coupled as a signal to a first attenuator.
The first attenuator is coupled to a first two-way splitter, the first two-way splitter receiving the signal from the array antenna through the first attenuator. The signal source two-way splitter splits the coherent reference phase signal into a reference signal which is carried as a signal to a second attenuator. The second attenuator is coupled to a second two-way splitter, the second two-way splitter receiving the signal from the signal source two-way splitter through the second attenuator.
From this point onward, the system in the receive mode is identical to the system in the transmit mode, starting with the previous description for the first and second mixers, and continuing with the first and second low pass filters, the quadrature base-band component signal, the in-phase base-band component signal, the processor, the recorder and the beam steering controller, and so will not be repeated here.
With regard to the method of the present invention, the method applies to calibrating at least one element in either a transmit array antenna or a receive array antenna. The method comprises the steps of: forming a difference null with errors for at least one element being calibrated in phase state n, thereby yielding the in-phase base-band component, In, and the quadrature base-band component, Qn, of the difference null; estimating the difference null without the element being calibrated by averaging over N phase states, by solving for
xe2x80x83xcex5=1/Nxcexa3(In+jQn)xe2x80x83xe2x80x83(1);
estimating the field of the element in phase state n by subtracting the null without the element from the null with the element; calculating field, En, of the element in phase state n by solving:
En=(In+jQn)xe2x88x92xcex5=Xn+jynxe2x80x83xe2x80x83(2);
computing the absolute value of the estimate of the field of the element in phase state n, |En|, and the phase of the estimate of the field of the element in phase state n, arctan(yn/xn); and processing results of the calibration of the element being calibrated upon determining that all desired array elements have been measured.
For either the system in the transmit mode or in the receive mode, or the method which can be applied to either the transmit mode or the receive mode, the array antenna can be a phased array antenna or an active array antenna. Also, the data representing the calibration reference values of at least one element of the array can be illustrated using a printer and/or a display unit.